This invention relates to an improved method of determining porosity characteristics of materials where the material to be studied is saturated with a liquid, subjected to a temperature increase whereby at least some liquid evaporates, and the resulting change in mass of the material is measured to analyze porosity of the material.
It is desirable to determine the porosity characteristics of certain materials since the extent of porosity influences other properties and assists in determining the usefulness of such materials. Porosity affects such things as the nature of strength, thermal and electrical conductibility, weatherability, permeability, and optical transparency of a material.
Some conventional methods used to determine porosity characteristics include electron microscopy, gas adsorption, mercury porosimetry, and phase transition porosimetry.
Electron microscopy involves direct observation of the pores of the material. While it is precise, it is not suitable for routine determination and rests upon the subjective determination of the operator of the instrument. It also has the disadvantage of involving the use of an expensive instrument to ascertain porosity.
Gas adsorption measures the size of pores by the adsorption and condensation of vapors in the pore space. Amount of vapor condensed in the pores at a given vapor pressure has been found to be related to pore volume, and pore size distribution is determined in this way. It has limited application, since it can be used only with material containing pores in the mesospore range of 3 nm (nanometer) to 50 nm. It is tedious, time consuming, and, as with electron microscopy, is also expensive to perform.
Use of a mercury porosimeter is one of the most frequently applied methods of determining porosity. Mercury is forced into the pores of the material and the volume of mercury which enters the pores is measured in relation to the pressure applied. The well-known Washburn equation, which can be found at S. Lowell, Introduction to Powder Surface Area, pgs 181-89 (Wiley & Sons, 1979), then allows the calculation of the pore size distribution, whereby pressure applied and pore radius are related. It can be effectively used with pore sizes between 4 nm to several micrometers. Some disadvantages with mercury porosimeters are discussed in U.S. Pat. No. 4,453,398, Demirel, et al., issued June 12, 1984 and relating to an ice porosimeter.
Among the disadvantages of mercury porosimetry are uncertainties in the relationship between pressure and pore radius. Certain empirical relationships are utilized; however, certain assumptions have to be made to arrive at results which are translated into porositic characteristics. Parameters such as pore cross-section geometry, surface tension of liquid in pores, and contact angles with regard to liquid and pores must be utilized in this method.
The Washburn equation assumes pores are cylindrical with perfectly circular cross-sections, while in nature this does not occur. Further, surface tension and contact angle values also vary in nature, while the equation presumes these to be constant. In addition, the material to be measured must be evacuated, and then pressurized, which can destroy the pore structure of the sample. The cost is relatively high and a concern exists about the health hazards of working with mercury.
Phase transition porosimetry, as described in U.S. Pat. No. 4,453,398, uses the change in volume of water in the pores as the same is frozen and/or thawed. The water-saturated sample is placed in mercury, and temperature and volume changes are measured as the material is frozen and/or melted. Pore water is expelled as the ice melts and changes in volume.
This device and process, while workable, can be effectively utilized only in material with pore size ranges of 2 nm to 100 nm, and is not suitable for use in measuring materials with large-size pores. The volume change is not large and is difficult to measure. This process also involves the use of the hazardous material mercury.
Accordingly, it is an object of the present invention to provide for determination of porosity characteristics of a material by measuring the change in mass of the material as liquid saturated in its pores evaporates.
A further objective of the invention is to provide for an accurate means of determining porosity characteristics of a material.
Another objective is to provide a means and method of determining porosity characteristics that does not require high pressures, evacuation or drying of the material which may destroy the pore structure.
Yet another objective is to provide a simpler and safer method of determining porosity characteristics which does not require high pressure or the use of hazardous vapors or materials.
Another objective is to provide a means of determining porosity characteristics of material which is rugged, efficient, and less expensive than other means, and which has wide ranging applicability to a variety of materials with a variety of different pore sizes, structures or shapes.
These and other objects, features, and advantages of the present invention will become apparent with reference to the accompanying specification and claims.